Production of transferrin receptors by Histophilus ovis: three of five strains require two signals

Five strains of Histophilus ovis (9L, 642A, 714, 5688T, and 3384Y) were inv estigated with respect to iron acquisition. All strains used ovine, bovine, and goat transferrins (Tfs), but not porcine or human Tfs, as iron sources for growth. In solid phase binding assays, total membranes from only two ( 9L and 642A) of the five strains, grown under iron-restricted conditions, w ere able to bind Tfs (ovine, bovine, and goat, but not porcine or human). H owever, when the organisms were grown under iron-restricted conditions in t he presence of bovine transferrin (Tf), total membranes from all strains ex hibited Tf binding (as above); competition experiments demonstrated that al l three Tfs (ovine, bovine, and goat) were bound by the same receptor(s). M embranes from organisms grown under iron-replete conditions in the presence or absence of bovine Tf failed to bind any of the test Tfs. An affinity-is olation procedure allowed the isolation of two putative Tf-binding polypept ides (78 and 66 kDa) from total membranes of strains 9L and 642A grown unde r iron-restricted conditions, and from membranes of all strains if the grow th medium also contained Tf. It is concluded that all strains tested acquir e Tf-bound iron by means of siderophore-independent mechanisms involving su rface receptors analogous to the Tf-binding proteins (TbpA and TbpB) found in comparable organisms; although iron restriction alone is sufficient to p romote the expression of these proteins by strains 9L and 642A, their produ ction by strains 714, 5688T, and 3384Y appears to require two signals, iron restriction and the presence of Tf.